Change meaning of nstime

I've been using it like this, any objections if we change the definition? Your epoch can be the last timestamp second making it as it was.

Author: Affie

src/entities/DFGVariable.jl

@@ -218,7 +218,7 @@ Base.@kwdef struct DFGVariable{T<:InferenceVariable} <: AbstractDFGVariable
     """Variable timestamp.
     Accessors: [`getTimestamp`](@ref), [`setTimestamp`](@ref)"""
     timestamp::ZonedDateTime
-    """Nano second time, for more resolution on timestamp (only subsecond information)"""
+    """Nanoseconds since a user-understood epoch (i.e unix epoch, robot boot time, etc.)"""
     nstime::Nanosecond
     """Variable tags, e.g [:POSE, :VARIABLE, and :LANDMARK].
     Accessors: [`getTags`](@ref), [`mergeTags!`](@ref), and [`removeTags!`](@ref)"""